My bad, you don’t actually need to mute the master/stereo out tracks of Logic. Just don’t send anything to them
In this screenshot I set it up so only “Print” (the track where I record my mixdown from B6’s master bus) goes to USB 15-16 (ext 2) of the B6. In Logic I mad this happens through Bus 1 (named Monitor).
The use of a bus is optional I think, I could just set print - or any set of Logic tracks - to output to 15-16 directly.
Note that in my experience these assignments are not related to the mac’s system audio output (global OS setting for all other apps).
I hope this helps!